This invention relates to lithographic printing forms and to precursors thereto, having aluminum supports bearing imagable coatings. The invention relates further to the manufacture of such precursors, and their use.
Typical lithographic printing form precursors are prepared as follows.
An aluminum sheet is subjected to a graining or roughening treatment. This may be a mechanical graining treatment, for example brush graining or ball graining, or an electrograining treatment (also called electrochemical etching or electrochemical roughening) in a mineral acid. The sheet is then anodised, to provide a hard hydrophilic surface, which has a microporous xe2x80x9choneycombxe2x80x9d structure. Anodising may typically take place in a sulphuric acid or phosphoric acid electrolyte. A post-anodic treatment (PAT) is then carried out, using, for example, a silicate or a phosphate composition. Subsequently a different composition, containing a polymeric substance, is applied in a liquid form, a solvent being removed therefrom to leave the imagable coating as a dry film on the aluminum sheet, which may be cut into individual lithographic printing form precursors. The resultant precursors may be imaged and developed, to provide the lithographic printing forms which are used for printing. During development, portions of the coating are selectively removed. In positive working systems portions which were exposed are removed. In negative working systems portions which were not exposed are removed. In most systems, whether positive or negative, the remaining portions of the coating are preferentially ink-accepting.
Many coatings contain dyes and these may be employed for several distinct reasons. A dye may alter the properties of the polymeric substance, for example by rendering it insoluble in a developer, but such that after imaging, the coating is soluble in the developer. Examples of such dyes are given in WO 97/39894, incorporated herein by reference. A dye may function as an absorber of imaging radiation, either as a xe2x80x9cspectral sensitizerxe2x80x9d to emit radiation of a different wavelength which triggers a desired chemical reaction to alter the properties of the coating, or as a compound which converts the absorbed radiation to heat, which alters the properties of the coating. Examples of dyes functioning as spectral sensitizers are given, for example, in U.S. Pat. No. 5,200,292, incorporated herein by reference. Examples of dyes converting imaging radiation to heat are given, in PCT/GB97/39894. Additionally, a dye colors a coating such that after development a positive or negative image can be seen on the printing plate. From this the printer can gain an impression of whether imaging and development has been successful and can identify and correct faults.
If the PAT step is not carried out any dye present in the coating may be seen in the regions from which the portions of the coating are removed on development; the dye may form an absorbed or residual layer on the anodised surface. This is undesirable as it reduces the contrast between exposed and unexposed portions of the printing plates and makes it harder for the printer to determine whether imaging and development has been successful, and to identify and correct faults. Furthermore dye which is present in uncoated areas may attract ink and cause poor printing performance.
It has also found that when a pigment is present rather than a dye, it is not necessary to effect a PAT step.
This invention is directed to a method of preparing a lithographic printing form precursor having an imagable coating on an aluminum support, the method comprising:
i. anodising an aluminum sheet (which is to serve as the support); and
ii. without having effected a chemical treatment step after the anodizing step, applying a composition comprising a polymeric substance to the anodised surface of the aluminum sheet and drying the composition to form the imagable coating thereon, wherein the coating contains a pigment and does not contain a dye.
This invention is also directed to a lithographic printing form precursor prepared by the above-described method, a method of making a lithographic printing form which comprises imaging and developing such a precursor, and a lithographic printing form obtained by imaging and developing such a precursor.